when I bought my coupe back in 07, all city driving I used to get between 400-500km to the tank, now I'm getting just over 300km, and I don't drive aggressively at all, in the last year I've taken it past 3k rpm maybe three times tops.
What you need is a ecu reflash to re calibrate a few things, there are a bunch of good tools you will find on
http://g35driver.com/forums/
I'm currently looking at doing the same thing before summer myself, shops like Speed Syndicate will be able to do the actual flash for you